Fluorescent analytical radiation source for producing soft x-rays and secondary electrons

ABSTRACT

A fluorescent analytical radiation source concurrently producing fluorescent soft X-ray and secondary electron emissions with high efficiency provided with means for selective energy cut off of the secondary electrons in order to isolate secondary electron emission characteristic of specific elements under analysis.

O United States Patent 1111 3,

[72] Inventors Robert D. Davies; [5 6] References Cited Heribert K. Herglotz, Wilmington, Del. UNITED STATES PATENTS [211 P 832,562 3,374,346 3/1968 Watanabe 250/49.5 1 [221 Filed June 1969 3,461,306 8/1969 Stout et al 250/49.5(8) [45 I Patented Mar. 2,1971 [73] Assignee E. I. du Pont de Nemours and Company Primary Borchelt Wilmington, Del. Assistant ExammerA. L. Bll'Cll Attorney-Harry J. McCauley [54] FLUORESCENT ANALYTICAL RADIATION SOURCE FOR PRODUCING SOFT X-RAYS AND SECONDARY ELECTRONS sclalmsADmwmg Figs ABSTRACT: A fluorescent analytical radiation source con- [52] U.S. Cl 250/515, currently producing fluorescent soft X-ray and secondary 250/495, 313/55 electron emissions with high efficiency provided with means [51] Int. Cl. G0ln 23/22 for selective energy out off of the secondary electrons in order [50] Field of Search 250/495 to isolate secondary electron emission characteristic of specific elements under analysis.

PATENTED "AR 2 I97! SHEET 1 [IF 3 km m QQN a 7&1? Q

INVENTOBS BObsrt 0. Davies Heriberl IEHczy Z012 BY/7,

M m (AM a ATTORNEY FLUORESCENT ANALYTICAL RADIATION SOURCE FOR PRODUCING SOFT X-RAYS AND SECONDARY ELECTRONS BRIEF SUMMARY OF THE INVENTION Generally, this invention comprises a fluorescent analytical radiation source for simultaneously producing fluorescent soft X-ray and secondary electron emissions characteristic of a sample in analysis comprising, in combination, within an evacuated housing, a centrally apertured, cooled metallic X- ray anode plate formed with a concave rotationally symmetrical electron impingement target surface, a cooled metallic cathode block provided with an inverted trough-form annular shield ring disposed coaxially with respect to the aperture of the anode plate with the shield ring and cathode block in confrontation with the anode target surface in close adjacency but out of contact therewith, a cooled metallic sample holder attached to the cathode block provided with a flat samplemounting surface facing the anode plate disposed coaxially with respect to the aperture and the annular shield ring in a location receiving primary X-ray emission from an annular area of the target surface, a high emission area cathode filament provided with electron-producing heating. means concentrically disposed with respect to the sample holder back of the annular shield ring, an electrical voltage source of X-ray excitation magnitude connected across the anode plate and the cathode block, and means maintaining an electrostatic guiding-condensing and electron-retarding field between the aperture and the sample holder limiting the kinetic energy of secondary electrons emerging from the aperture to a preselected cutoff.

DRAWINGS FIG. 1 is a schematic side elevation cross-sectional view of a preferred embodiment of radiation source according to this invention, including plots of equipotential lines between anode and cathode and adjacent the electron-retarding element, together with path limits of bombarding electron fluxes;

FIG. 2 is a partially schematic side elevation cross-sectional view of the cathode structure, sample holder and associated cooling means for apparatus similar to that of FIG. 1, except that the anode is grounded and the cathode is maintained at negative potential level;

FIG. 3 is a schematic side elevation view, with evacuated enclosure housing omitted, of the radiation source of this invention according to FIG. I provided with associated analytical means for the fluorescent soft X-ray and secondary electron emission outputs of the apparatus; and

FIG. 4 is a plot on voltage-time axes of a tuning fork driving signal and a typical detected signal obtained during operation of the analytical means for the fluorescent soft X-rays shown in FIG. 3.

Currently, spectroscopic analysis of chemical compounds has been confined to the obtainment of (l) bulk information by conventional X-ray analysis and (2), more recently, electron spectroscopy for chemical analysis, frequently abbreviated ESCA, which yields information on atomic and molecular structure to a depth of, typically, 100 A., and also as to the surface state, e.g., composition and bonding. This has not only required more time than a single analysis but, more important, it has not been possible to examine the identical area of the sample under the same environmental conditions during both analyses, which is disadvantageous.

This invention provides an efficient concurrent source for both fluorescent soft X-rays and secondary electron emissions which enables simultaneous conduct of both types of analyses on the identical sample area and under the same environmental conditions.

A particularly convenient analytical arrangement is to utilize the radiation source of this invention with the improved spectrograph of U.S. Pat. No. 3,418,466, assigned to common assignee, for the conduct of the X-ray analysis while concomitantly utilizing conventional means for energy analysis of the secondary electrons after selective cutoff of the latter within the source in order to sharpen the analytical perception.

Fluorescent X-ray sources of the prior art are of low inherent brightness compared to direct electron-excited sources. The sources employ an X-ray generator which bombards a specimen, thereby exciting characteristic Xray fluorescence. Concurrently, a variety of secondary electron emissions are produced which are referred to as photoelectrons, Auger electrons, shake-off electrons, and by other names.

In the usual X-ray analyzer of the prior art, the number of secondary electrons diffusing toward the detection system is negligibly small. If, however, a high electron flux is produced and channelled in the direction of the X-ray beam, the secondary electron emission may obscure the X-ray signal, so that suppressing or trapping elements need to be employed to permit meaningful X-ray analysis. It is an important feature of the source of this invention that, because of the unique geometry and electron-focusing arrangement, the secondary electrons are made to contribute to the total information respecting the sample, the suppressing element employed being an essential component of the source itself.

Referring to FIG. 1, a preferred embodiment of our source utilizes a metallic cylindrical anode element 10, watercooled by means not detailed, shaped internally to a frustoconical X- ray target surface 10a disposed toward the metallic cathode block assembly, also water-cooled as hereinafter detailed in FIG. 2, denoted generally at 11. Anode 10 is drilled centrally to present an aperture 10b for exiting analytical radiation and cathode block assembly 11 is provided with an upraised flat metallic sample holder 12 disposed coaxially with respect to aperture 10b (typically, 0.75" therefrom) facing anode element 10. The upper periphery of the cathode block is formed to a frustoconical shape matching that of the anode and underlying the latter. Thus, the entire construction is rotationally symmetrical about vertical axis A-A.

Cathode filament 13 is a stiff circular form helically wound electron-emissive wire, such as tungsten, disposed coaxially with respect to aperture 10b and sample holder 12, filament 13 being conveniently supported from cathode block assembly 11 as hereinafter described with reference to FIG. 2, but which can be independently supported if desired. Cathode filament 13 is provided with conventional electron-producing means such as a voltage supply or heater not detailed in FIG. I.

An inverted trough-form annular shield ring 20 is conveniently supported radially inboard from the peripheral portion of the cathode to overlie cathode filament l3 and thereby shield anode 10 from direct contamination by evaporative emission of tungsten from the filament.

A depending apertured metal cup 16 is mounted within aperture 10b, with its own aperture 16a located coaxial with axis of symmetry A-A. Annular metal electron guiding-condensing and retarding ring electrode 14 (typically, 0.25" long with a bore dia. of 0.080") is coaxially disposed within cup 16 and insulated therefrom by ring insulator l9, electrode 14 thus constituting the analytical radiation exit from the source. Ring electrode 14 is formed at its inboard end with an outwardly tapered focusing bevel 14a.

For purposes of electrical shielding and general simplicity of electrical connections, it is preferred that anode 10 be maintained at a positive potential of, typically, l0 Kv, whereas the cathode is then grounded. However, if desired, the anode can be grounded as described with reference to FIG. 2 and the cathode carried at a suitable negative potential difference between these components in order to appropriately accelerate the bombarding electrons emitted by cathode filament 13.

In FIG. 1, the potential source 18 comprises two independently adjustable sections 18a and 18b, both connected to ground via opposite polarity terminals and conductor 180, the

positive terminal of section 18a being connected to anode via conductor 18d and the negative terminal of section 18b being connected to ring electrode 14 via conductor 18c. This permits selection of the potential carried on ring electrode 14 to maintain a field counter to that hereinafter described between the source anode and cathode. This ring electrode field is depicted by pattern f, as obtained from semiconductive paper plots, imposing a relatively strong electrostatic guidingcondensing and electron-retarding action exerted on the secondary electron emission from the source.

The electron flux from cathode filament 13 to target surface 10a is denoted in broken line crescentlike cross section a in FIG. 1, it being understood that this flux is substantially uniform around the full 360 circumference of the filament. Considered in two-dimensional representation as shown in FIG. 1, the electron flux is constrained to a course generally orthogonal to the equipotential lines e maintained within the anode-cathode interspace, typical potential values of representative lines being denoted as obtained from semiconductive paper plots for the example source.

The accelerated electron flux bombards a generally annular area b of target surface 10a and generates primary X-ray radiation substantially uniformly around the entire 360 circumference directed towards the sample-mounting face 12a of sample holder 12 within cross section limit lines denoted d in FIG. 1. Thus, a specimen mounted on face 12a is subjected to primary X-radiation and emits fluorescent soft X-radiation characteristic of its composition as well as a very copious supply of secondary electrons, which, together, constitute the exiting analytical radiation from the source of this invention. The secondary electron portion of this analytical radiation is accelerated by the anode-cathode potential differential and relatively concentrated beam exits along axis A-A to the detection system hereinafter described. Aperture 16A serves to collimate the secondary electron output delivered.

However, field f, counter as it is to field e, permits regulable deceleration of the secondary electron emission, so as to cut off all such emission below a preselected final velocity level while passing the remainder at relatively low residual velocity through ring electrode 14.

The guiding-condensing action of ring electrode 14 is secured by the influence it exerts upon the electrostatic field existing between itself and the lower portion of cup 16. That is, the equipotential lines f, as plotted between these two elements, are determined primarily by the ratio of the element diameters and their spacing, as well as the potential difference maintained thereacross. The potential gradients represented are such that the electrons on first entering this field are ini tially diverged until, on entering the upper half of the retarding field, they experience a condensing effect together with a retardation before they exit from the source.

Referring to FIG. 3, a preferred arrangement for concurrent sample analysis based on both fluorescent X-radiation and secondary electron emission can employ the source of this invention, denoted generally at 50, which discharges its exiting radiation from ring electrode 14 across the entrance end of a conventional hemispherical electrostatic deflector 28 having the two shell elements of potentials denoted. A typical design of deflector 28 is that described in the article entitled Electron Monochromator Design, Review of Scientific Instruments, Vol. 28 (Jan. l967 pp. 103 l l 1. However, other designs can be utilized, such as those employing the Wien filter, as described by Boersch, I-I., Geiger J. and Hellwig, H., in Physics Letters, Vol. 3, No. 2 (1962) pp. 64-66. Deflector 28 effectively isolates exiting secondary electron emission by directing it along a course g terminating at conventional detector 31. Substantially all of the secondary electron emission above a preselected velocity impinges on the walls of deflector 28, so that the emission fraction having a given velocity that is discharged from exit 29 is a measure of a sample characteristic under investigation.

Detector 31 incorporates a collector 31a upon which the secondary electron emission emerging from exit 29 impinges,

this furnishing the input to an operational amplifier 31b provided with resistive-capacitive negative feedback, typically a Model 301 amplifier marketed by Analog Devices Corporation. The entire assembly is mounted within an electrically grounded evacuated housing with output signal transmitted via lead 310 to an oscilloscope, not shown.

Simultaneously, the fluorescent X-radiation, which is completely. unaffected by the electrostatic field of deflector 28, passes through an aperture 30 disposed in line with ring electrode 14 and impinges on X-ray dispersing element 26, this course being denoted by broken line trace h.

As hereinbefore mentioned, the fluorescent X-radiation is analyzed by the apparatus taught in US Pat. No. 3,418,466 of common assignee. This embodies a detector 27 mounted on a focusing Rowland circle, not detailed, receiving preselected X-radiation reflected from dispersing element 26, and preferably also incorporates, as a new feature, a tuning-fork vibrator 32 reciprocating a pinhole aperture 34 transverse the X-radiation beam delivered by source 50 as tines 33 oscillate. The entire apparatus is mounted within an evacuated housing,

. not detailed.

As described in US. Pat. No. 3,418,466 the X-ray beam impinging on dispersing element 26 is caused to pass through the critical angle, such traversal being effected automatically by reciprocatory pinhole aperture 34 which, typically, deflects the radiation by as much as 1 for a dispersing element having a length of 0.5l". This causes detector 27 to receive reflected X-radiation of alternating strength as a result of the tuning-fork vibration, so that the detector generates an electrical AC signal having a magnitude proportional to the concentration of the element in the specimen fluorescing with a characteristic wavelength. Detector 27 can be an open-window photoelectron multiplier with output coupled to an amplifier, not shown, and band-pass filter, not shown, and thence displayed on an oscilloscope, also not shown. A particular advantage of the system described is that the phase of the detector signal can be continuously monitored by triggering the oscilloscope via lead 32a with the tuning-fork driving signal introduced via lead 32b.

A second embodiment of the source of this invention is shown as regards the cathode assembly solely in FIG. 2, the arrangement for cooling, which is also utilized for the FIG. 1 embodiment, being here detailed.

In this construction the anode is maintained at ground potential whereas the cathode assembly 11 is carried at a negative potential, e.g., l0 kv., 30 relative X-ray excitation magnitude. Shield ring 20 is formed integral with the cathode at an angle of inclination of approximately 30 with the horizontal and overhangs cathode filament 13, which is mounted in encirclement of sample holder 12 by a plurality of electrically conductive cantilever supports two of which, 13a

and 13b, are shown attached at their radially outboard ends to the cathode assembly. Support 13a is electrically insulated from the a cathode assembly and connected in electrical circuit with supply lead 38, also insulated from the cathode assembly by sleeve 38a, which furnishes the electron fluxproducing energy for filament 13. A current-return terminal (not shown) is also attached to the cathode assembly.

The underside of holder 12' is recessed axially, as is the central region 39 of the cathode assembly to provide a chamber for colling water circulated therethrough via metal tubes 40 and 41. These tubes, together with lead 38, are out through a drilled metal cap 42 supported on a depending sleeve insulator 43 which, in turn, is surrounded by an annular insulation disc 44 preventing surface tracking. Sleeve insulator 43 is supported, in turn, at its upper end by attachment to grounded metal flange 45 constituting part of the evacuated housing.

Cooling water tubes 40 and 41, both at high potential level, are insulated from water supply piping by lengths of polymeric, e. g., polyethylene, tubing, typically each 75 ft. long, not shown, and tube 41 is fitted with a clamp-on electrical supply lead such as that denoted at 46.

In operation, equipotential lines e (FIG. 1) form a relatively constant gradient, as demonstrated by the general parallelism of the lines with the anode and sample support surfaces apparent in FIG. 1. Moreover, in accordance with the laws applicable to electrostatic fields, the electron trajectories of the primary bombarding flux a are nearly orthogonal to the equipotential lines e so long as initial velocities are small compared to velocities due to energy gain from the field, as is the case with the construction taught. Under these conditions a typical X-ray target area of length b, which can be approximately one-fourth inch, is impacted producing primary X- radiation throughout a full 360 angle as to which there is very even distribution of radiation on the sample mounted on surface 12a.

The emission of fluorescent characteristic soft X-rays and secondary electrons produced by excitation of the sample occurs in a random manner, those X-rays traveling upward through aperture 16a being reduced to an apparent point source by the fine pinhole 34 reciprocated transverse the beam course h. The paraxial secondary electrons are selectively decelerated and guided out of the upper end of ring electrode 14 into deflector 28 and analyzed as hereinbefore described.

Since the specimen on surface 12a is cooled at all times, X- radiation impingement thereon is not destructive of the sample substance. At the same time, due to the geometry of the source and the specific design and relative location of its components, the strength of primary X-radiation brought to bear on surface 12a markedly exceeds that heretofore obtained with fluorescent sources utilized for the production of soft X- rays.

EXAMPLE 1 The theoretical critical angle for carbon fluorescent X- radiation incident on a pin parafiin mirror employed as dispersing element 26, FIG. 3, is approximately 3 50'. Only in the vicinity of this critical angle should a small variation of incident angle yield a signal characteristic of carbon since, for 9 6 the radiation is all reflected, whereas for 9 9 it passes into the paraffin.

To check whether a critical angle could be detected for a graphite sample, the paraffin mirror 26 was originally set at 3. A small signal of the same frequency as tuning-fork vibrator 32, but 180 out of phase with the driving signal, was obtained. The incident angle was then increased to 4, by slightly moving mirror 26 along the Rowland circle and the procedure repeated. In this case a large periodic signal in phase with the driving signal was obtained, as shown in broken line representation in FIG. 4. With further increase of the incident angle to 5, the signal reverted to its former state, i.e., small and 180 out of phase with the driving signal. This confirmed the presence of a critical angle pronounced enough to base analysis thereon.

The graphite sample was replaced with an aluminum specimen and each of the foregoing measurements was repeated. No in-phase signal could be detected at any incident angle within the 3-5 incident angle range.

Resubstitution of a graphite sample immediately gave the large in-phase signal characteristic of carbon.

The smaller out-of-phase signal can be explained by a variation in the stereo angle of the X-ray beam striking mirror 26 as a function of incident angle.

There are also several modulating signals present due to noise and fluctuations in emission current. These effects can, however, be minimized by reducing the amplitude of the vibration about the critical angle and by stabilizing both the filament current and the emission current.

The presence of nitrogen in a different sample was similarly confirmed using a lithium fluoride mirror, yielding a critical angle of approximately 4 30 as predicted theoretically.

EXAMPLE 2 This example verifies the adaptability of the source of this invention as an analytical means for use in secondary electron spectroscopy.

First, it is known that an unknown specimen can be determined by energy analysis of the I(-, I..- or M-electrons ejected from their orbits in the atoms making up the X-ray irradiated portion of the specimen. Moreover, a sample bombarded by energetic X-rays emits two distinct types of electrons, namely, photoelectrons and Auger electrons. In illustration, an oxygen-containing sample was irradiated by AlKa X-rays ,of 1487 ev. energy. The X-rays can eject a K electron from the oxygen, the K excitation potential being 532 ev. The kinetic energy of the emitted electron is thus 955 ev. the difference between the energy of the ejected electron and the energy by of the impinging X-rays. An electron from the L-shell can fall into the vacancy in the K-shell, with the accompanying emission of a 525 ev. photon (the difference in energy being the 7 ev. binding energy of the L-shell). There is an approximately 99 percent chance that the photon will be internally absorbed to eject a second electron from the L-shell, this electron being commonly known as the Auger electron, having a kinetic energy of5 l 8 ev.

For substantially monochromatic primary X-rays, the energy of both the photoelectron and the Auger electron is characteristic of the emitting element and the bonding state of the element.

To detect the difference in energy of the electrons emitted from elements in the second row of the Periodic Table, the following experiment was conducted using retarding fields.

A retarding field ring electrode 14 was installed in the fluorescent source of FIG. 1 and connected to a variable negative voltage supply 18. The exit aperture diameter of ring electrode 14 was 0.08 and the electrode was mounted within aperture 1011 by frictional fit inside an annular A1 0 disc of approximately %-iI1Ch diameter. The source was operated at 6 kv. anode-cathode differential voltage.

The ring electrode 14 voltage required to reduce the electron signal to a minimum (i.e., the cutoff voltage) was noted in a preliminary experiment for each of four specimens: (1) polypropylene, (2) boron nitride, (3) quartz and (4) rock salt, containing, respectively, the elements (1) carbon, (2) nitrogen, (3) oxygen and (4) sodium. The results are tabulated as follows:

Cut-off" Element Auger Voltage Analyzed Electron (arbitrary Sample for Energy (eV) units) Polypropylene C 270 2.7(i-0. l) Boron nitride N 380 3.0(:(). l) Quartz 0 520 3.2(-' -0. 1) Rock salt Na 1000 3.8(i0.1)

The error ranges reported refer to meter readings only, due to the difficulty in determining the precise -cutoff" point as the intensity decreases with increasing negative voltage.

The lower the negative cutoff voltage required to retard the electrons sufficiently to contain them within the source, the lower the electron energy. It can be seen from the data that the progressively increasing Auger energy from carbon through sodium corresponds to an accompanying increase in the cutoff voltage. It is concluded that, in each instance, the decrease in electron signal is due to the trapping of the Auger electrons characteristic of each of the four elements reported.

The example anode construction employs a frustoconical X-ray target surface 10a; however, other configurations, such as spherical, parabolic or the likecan be utilized equally-well and, in some instances, even more efiiciently. Moreover, a wide variety of electrical circuits and cooling systems can be utilized for cathode and anode elements, so that no particular limitations with respect to these features are implied.

A great variety of cathode filament constructions and configurations are usable including wire elements bent at close turn spacings in flat horizontal or vertical spirals, so as to present a high bombarding electron emission area.

Moreover, the source of this invention can be very advantageously employed solely as a secondary electron emission source, entirely apart from its use as a conjoint X-ray source herein described.

From the foregoing, it will be understood that this invention is subject to relatively wide modification without departure from its essential spirit and it is, accordingly, intended to be limited only by the scope of the following claims.

We claim:

1. A fluorescent analytical radiation source for simultaneously producing fluorescent soft X-ray and secondary electron emissions characteristic of a sample in analysis comprising, in combination, within an evacuated housing, a centrally apertured cooled metallic X-ray anode plate formed with a con cave rotationally symmetrical electron impingement target surface, a cooled metallic cathode block provided with an inverted trough-form annular shield ring disposed coaxially with respect to the aperture of said anode plate with said shield ring and cathode block in confrontation with said target surface in close adjacency but out of contact therewith, a cooled metallic sample holder attached to said cathode block provided with a flat sample-mounting surface facing said anode plate disposed coaxially with respect to said aperture and said annular shield ring in a location receiving primary X-ray emission from an annular area of said target surface, a high emission area cathode filament provided with electron-producing heating means concentrically disposed with respect to said sample holder back of said annular shield ring, an electrical voltage source of X-ray excitation magnitude connected across said anode plate and said cathode block, means maintaining an electrostatic guiding-condensing and electron-retarding field between said aperture and said sample holder limiting the kinetic energy of secondary electrons emerging from said aperture to a preselected cutoff.

2. A fluorescent analytical radiation source for simultaneously producing fluorescent soft X-ray and secondary electron emissions characteristic of a sample in analysis according to claim 1 wherein said X-ray anode plate is formed with a concave rotationally symmetrical electron impingement target surface of frustoconical configuration.

3. A fluorescent analytical radiation source for simultaneously producing fluorescent soft X-ray and secondary electron emissions characteristic of a sample in analysis according to claim 1 where said high emission area cathode filament is a helically wound element. 

2. A fluorescent analytical radiation source for simulTaneously producing fluorescent soft X-ray and secondary electron emissions characteristic of a sample in analysis according to claim 1 wherein said X-ray anode plate is formed with a concave rotationally symmetrical electron impingement target surface of frustoconical configuration.
 3. A fluorescent analytical radiation source for simultaneously producing fluorescent soft X-ray and secondary electron emissions characteristic of a sample in analysis according to claim 1 where said high emission area cathode filament is a helically wound element. 